A cam sprocket is attached to one end in axial direction of an engine cam shaft, and it is necessary to attach the cam sprocket to the cam shaft while positioning it such that the center of curvature of a virtual circle circumscribing its external teeth matches the rotation axis of the cam shaft.
In order to perform this positioning, it is known to provide a cylindrical portion jutting out to one side in axial direction from the radially inner side of the cam sprocket, and to fit one end in axial direction of the engine cam shaft into this cylindrical portion.
The bottom wall portion of this cylindrical portion is provided with a through hole for a bolt, and by inserting a bolt through this through hole and screwing it to a female screw hole on one end face in axial direction of the cam shaft, the cam sprocket is fixed to the cam shaft.
A conventional cam sprocket provided with such a cylindrical portion is ordinarily fabricated by a sintering process. That is to say, it is fabricated by filling a powdered alloy into a casting die and sintering it. Even though the degree of freedom for shapes in this sintering process is high, and it is possible to make the depth of the cylindrical portion of the cam sprocket as deep as practicable, it has been pointed out that this process is time-consuming so that the manufacturing costs are high.
On the other hand, with the goal of lowering the manufacturing costs, cam sprockets have come to be formed in recent years by stamping. Sprockets made of such stamped components are disclosed for example in PTLs 1 and 2.
PTL 1 discloses a sprocket with a configuration in which two sprocket halves fabricated by stamping are joined together to face the surface. To fabricate the sprocket halves, first, an iron sheet is blanked to form teeth along its circumference and an axial hole by stamping, and punching to one side is performed to provide a boss portion around the axial hole.
PTL 2 discloses a single cam sprocket fabricated by stamping. This cam sprocket is made by a step of stamp a center portion of a disk-shaped steel sheet to form a groove on the bottom, a step of increasing the wall thickness of the outer circumference in axial direction by rotating roll shaping an outer circumference of the disk-shaped steel sheet, a step of forming a marker hole and weight-reducing holes in the disk-shaped steel sheet by punching, a step of lathing the inner diameter of the marker hole, a step of reaming the marker hole, a step of cutting teeth into the outer circumference of the disk-shaped steel sheet, followed by subjecting the disk-shaped steel sheet to carburizing and quenching, shot blasting, and finishing the inner diameter of the marker hole, for example.